The wide band-gap of SiC (irrespective of polytype) results in a very high critical electrical field before avalanche breakdown. This critical field is roughly ten times that of Si and as a consequence one tenth of the thickness of material is needed to sustain a given voltage in the off state, which in turn leads to a much lower resistance in the on-state. This lower resistance reduces the power dissipation in the device resulting in overall system energy savings.
However, care must be taken when calculating the actual breakdown voltage of a real device since the theoretical critical electric fields assume an infinite planar junction. However, in the real case field intensification occurs around the edges of a diffused region and the breakdown voltage is reduced as a strong function of the radius of curvature of the structure. (see for example: J Baliga, Fundamentals of Power Semiconductor Devices, page 108).
This problem is well understood in the context of Si devices and solutions are found by adding further field relief structures around the edges of active junction (J Baliga, Fundamentals of Power Semiconductor Devices, page 130). Such a field relief structure is shown in FIG. 1. The field relief structures can gradually release the depletion region at the edge of the device. The voltage is supported between each pair of p+ rings, thus reducing the crowding of the electric field at the surface. The spacing and depth of the rings are designed such that the electric field peaks at the surface between each pair of rings are almost equal and the breakdown voltage of the final structure is significantly increased.
Junction Termination Extension (JTE) structures have also been proposed in Si based devices. Such a JTE structure is shown in FIG. 2. This structure uses multiple zones with the doping level decreasing from the active area towards the end of the termination. The effect is similar to that achieved by the field rings—that is the gradual release of the depletion region at the edge of the device. The voltage is ideally supported uniformly along the JTE zones.
However, SiC presents new challenges in this area, most importantly as mentioned above, the electric fields in SiC are very high and so this field intensification problem is much more severe. Consequently to extrapolate the Si solution above to SiC would result in very complex structures with much smaller dimensions to the extent of not being practical using normal production equipment.
Furthermore, the material itself presents problems; since dopant diffusion is negligible in SiC the deep, slowly curving diffused structures used in Si cannot be reproduced, and the alternative dopant introduction method of ion implantation by its very nature cannot give the same structure. In fact the only practical method of introducing localised doping in SiC is ion implantation, but again keeping implantation energies within the scope of production equipment results is rather shallow junctions of the order of 0.5 microns deep which exacerbates the whole problem.
It is an object of the present invention to address the problems above.